Generally, a known image forming apparatus includes a holder holding a plurality of photosensitive drums, and a pair of left and right metal frames each having a positioning portion for positioning the holder relative to a main body. In the known image forming apparatus, the holder has side plates, a front beam, and a rear beam. A positioning shaft is provided at a front end portion of the holder. A rear end portion of the holder contains notch portions. The positioning shaft is engaged in recessed portions formed at front ends of the left and right metal frames of the main body and the notch portions are engaged with a reference axial member fixed between the left and right metal frames of the main body such that the holder is positioned relative to the apparatus body. The positioning portion is provided on each metal frame having small dimensional change due to a change in temperature, improving the positional accuracy of the holder relative to the main body.